This invention relates in general to transmissions and transaxles and like machinery and more particularly to machinery of that character having bearings which are mounted to compensate for differential thermal expansion and contraction between cases and shafts.
In an effort to reduce the weight of their vehicles, automobile manufactures have turned to lightweight aluminum alloys for the cases of transmissions and transaxles. Yet the shafts which turn in these cases and carry the gears that transmit the torque remain of steel, obviously because steel has great strength and resists wear. A variety of bearing arrangements exist for mounting shafts in transmission and transaxle cases, but the most compact and durable utilize tapered roller bearings. In a typical in-line transmission, the input and output shafts are axially aligned and are confined at opposite ends of the case in two single row tapered roller bearings which, with respect to each other, are directly mounted, that is the large ends of the rollers for each bearing are presented inwardly toward the interior of the case and toward each other. Moreover, the input shaft has a pocket which receives the end of the output shaft, and here the output shaft is provided with another single row tapered roller bearing, known as a pocket bearing, which is also mounted directly with respect to the bearing for the output shaft. The input and output shafts carry gears which mesh with gears on a countershaft, and the countershaft at its ends is fitted with single row tapered roller bearings that are set into the case also in the direct configuration. A typical manual shift transaxle has an input shaft and a parallel countershaft which are likewise set in directly mounted tapered roller bearings that are fitted to the case, and the countershaft is in turn coupled through gearing with the differential shaft.
The tapered roller bearings carry extremely heavy loads for their size. Furthermore, they take axial or thrust loads as well as radial loads, and thus, a minimum number of bearings accommodate all of the loads to which the shafts are subjected. These characteristics serve to make transmissions equipped with tapered roller bearings highly compact.
Ideally, opposed tapered roller bearings should operate within an optimum setting range dictated by application requirements. Generally speaking, the objective is to minimize axial and radial free motion in the shafts, for this maximizes the bearing life, reduces noise, and improves gear mesh. The directly mounted bearings which support the aligned input and output shafts and the countershaft of the typical in-line transmission and the parallel input shaft, main shaft and differential shaft of the typical manual transaxle in effect capture those shafts axially. If the transmission or transaxle case were made from steel, like the shafts and bearings, the case and shafts and the bearings would expand similarly with temperature variations, and the settings of the bearings for each shaft would not change drastically over a wide range of temperatures. However, the aluminum alloys, from which many cases for the transmissions and transaxles of current manufacture are cast, have coefficients of thermal expansion greater than that of the steel from which the shafts and bearings are made. Assuming such a transmission or transaxle is assembled at room temperature with its directly mounted bearings in a condition of zero end play, the bearings will experience preload when the temperature drops, because the case contracts more than the shafts. By the same token, the bearings will experience end play as the temperature rises above room temperature, since the case expands more than the shafts. While the expansion and contraction of the tapered roller bearings, due to the geometry of the bearings, tends to offset some of the effects of the differential expansion and contraction between the case and shafts, it is not nearly enough to maintain bearing settings generally constant over a wide range of temperature. Excessive preload increases the effort required to shift gears, particularly when changing from a high gear to a lower gear. This is most likely to occur during cold start up. Excessive end play, on the other hand, decreases the size of the zones through which loads are transmitted in the bearings, and this reduces the life of the bearings. It may also increase gear noise. Since end play allows some radial and axial displacement of the shafts, it may also change the positions in which the gears mesh.
The transmission and the transaxle of the present invention have all of their shafts mounted on tapered roller bearings, and those bearings at the ends of the shafts are set into the transmission or transaxle case. The bearings compensate for differential thermal expansion and contraction between the shafts and the case. As a result, the bearings remain at a more uniform setting over a wide range of operating temperatures.